ST. LOUIS -- The last time Felix Doubront stood on the mound at Busch Stadium, it was Game 4 of the 2013 World Series and he earned the win in relief for Boston against St. Louis.

Doubront was in a Cubs uniform Saturday and starting, but the result was the same.

The left-hander pitched seven strong innings in his National League debut and Starlin Castro hit one of Chicago's three solo homers, sending the Cubs to a 5-1 victory over the slumping Cardinals in the opener of a day-night doubleheader.

"I was thinking about that, stepping on the mound and feeling the way I felt in the World Series last year," Doubront said. "This is my team now. I give everything 100 percent and try to win games. I want to stay here and do the best job that I can."

Doubront (1-0) scattered seven hits in his return from the disabled list. He had been out with a strained right calf since Aug. 1, two days after Chicago acquired him from Boston, where he was 2-4 this season.

"We were hoping he'd be very efficient today and he was," Chicago manager Rick Renteria said about Doubront's 89-pitch effort. "He was very impressive. That's a pretty good lineup. He did a nice job of containing them. He seemed very poised out there. We're very happy with his outing."

Logan Watkins and Chris Valaika also connected for Chicago in the makeup of a May 14 rainout. The last-place Cubs hit four home runs Friday, giving them seven in two games against the Cardinals.

"It's about the same thing we talked about yesterday," St. Louis manager Mike Matheny said. "It's a team that's really capitalizing on the home run. They've got a nice thing going over there."

It's not going so well for the Cardinals.

St. Louis has lost four in a row for the second time this season. The Cardinals began the day one game ahead of Atlanta and 1 1/2 up on Pittsburgh for the second NL wild card.

Chris Coghlan had a two-run single for the Cubs, who have won nine of 13.

Justin Masterson (2-3) gave up three homers and five earned runs in 4 1/3 innings for St. Louis. The right-hander was 4-6 when he was obtained from Cleveland on July 30.

Masterson has allowed four or more runs in five of his six starts for the Cardinals and pitched five innings or fewer in four of his starts.

Tyler Lyons struck out eight in 4 2/3 scoreless innings of relief for St. Louis.

The Cubs took a 2-0 lead in the second, when an out at the plate was overturned on instant replay. Doubront drew a walk to load the bases before Coghlan singled to right. Plate umpire Alan Porter initially called Valaika out at home on a throw from Randal Grichuk.

But a replay showed Valaika had his hand on the plate before he was tagged by catcher A.J. Pierzynski. Chicago challenged and the call was changed.

St. Louis got a run back in the second when Jhonny Peralta scored on a double play.

Watkins hit the first pitch of the fourth for his first major league home run. He started in place of Jorge Soler, who homered twice Friday.

"I'm trying to keep everything in perspective," Watkins said. "I expect this to happen. I've gone every level on the way here and worked hard and I've paid my dues. I've earned it. It's cool to hit a home run, but I expect to hit more in my career. Obviously, I'm very excited and happy. This is the first of hopefully many."

Two pitches later, Valaika sent a shot into the left-field bullpen, giving the Cubs a 4-1 lead.

"All of them have enough strength to hit it out of the ballpark," Renteria said. "They had good swings and good approaches. We're happy they did it. It was another good homer day for the Cubbies."

Castro led off the fifth with his 14th homer, matching his career high.

TRAINER'S ROOM

Cubs: 1B Anthony Rizzo, who was second in the NL with 30 homers, sat out again. Rizzo's lower back tightened during a rain delay Tuesday in Cincinnati. He has not played since and is expected to sit out Sunday as well. ... RHP Edwin Jackson (strained right strain) threw in the bullpen before the game. He has been on the disabled list since Aug. 21.

Cardinals: RHP Michael Wacha (right shoulder strain) will pitch Sunday in Tulsa for Double-A Springfield. Wacha went on the DL after his June 17 start.

UP NEXT

Cubs: LHP Tsuyoshi Wada (4-1, 2.56 ERA) was set to make his ninth big league start in the second game. In his previous outing Sunday against Baltimore, Wada took a no-hitter into the seventh before Steve Pearce hit a leadoff homer. ... LHP Travis Wood (8-11, 4.72) will start Sunday afternoon.

Cardinals: LHP Marco Gonzales (0-2, 7.07) was scheduled to pitch the nightcap. He was called up as the 26th man on the roster to make his fourth major league start and first since July 6. ... RHP John Lackey gets the ball Sunday.

ROSTER MOVES

Chicago optioned LHP Zac Rosscup to Triple-A Iowa to make room for Doubront on the 25-man roster. Rosscup remained available for both games Saturday as the Cubs' 26th man. He will not be available Sunday, but is expected back Monday when rosters expand.